The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for improved lung Fourier decomposition MRI.
Pulmonary proton MRI has long faced challenges of low signal-to-noise ratio (“SNR”) because a majority of the lung is gas with low proton density. A promising new approach to lung imaging with MRI acquires serial images during free tidal breathing. Each image from a different time point in the breathing cycle is registered to a reference image and the time series of each voxel is Fourier transformed. Peaks in the spectrum at the breathing and heart rate frequencies are used to identify the amplitude of regional proton density changes related to “ventilation” and “perfusion” respectively. This method is known as the lung Fourier decomposition method. In the spatial registration procedure, the position and size of each voxel is registered to a corresponding voxel in a reference image. The change in proton density at the breathing frequency is entirely attributed to the modulation of proton density due to changes in gas volume during ventilation. However, as lung volume changes due to inhalation or exhalation, proton density may be further changed due to a change in blood volume. For example, increasing lung volume may act to reduce proton density even further due to a decrease in blood volume secondary to the stretching of alveolar septa and compressive force on the capillary blood. Thus, conventional Fourier decomposition methods provide an inaccurate representation of proton density changes associated with ventilation.
In addition, conventional Fourier decomposition methods do not provide information about the ventilation dependent lung blood volume. The ability to obtain an image of this ventilation dependent blood volume via noninvasive means, such as MRI, has yet to be achieved. An image that depicts ventilation dependent blood volume has the potential to be a valuable noninvasive pulmonary function test that may have significant diagnostic value.
It would therefore be desirable to provide a system and a method that provides a decomposition of proton density changes observed at the breathing frequency that are due to true ventilation and to changes in blood volume that are caused by ventilation.